Thermal Mass Flow Meters

Thermal Flow meters are referred to in a variety of variations: thermal flow meters, thermal meters, thermal mass flow meters, calorimetric flow meters, and thermal dispersion flow meters. They all are referring to the same principle of operation.

Thermal mass flow meters operate on the principal that a flow stream produces a rate of heat transfer, where heat is removed and cooling takes place, that is proportional to the mass flow of the media. There are two elements to a typical thermal mass flowmeter, a heating element and a sensing element. The heating element is cooled by the flow and is detected by the sensing element. This difference is translated into a flow value.

To envision how this works, imagine you are standing outside in a t-shirt on a fall day. There is a light breeze blowing, it’s about 60 degrees and you are comfortable. Now imagine that the breeze turns into a strong wind. The increase in the wind speed begins to chill you because your skin is losing more heat. This is the same principle that thermal mass flow meters operate on. The stronger the wind, the less heat you retain. With thermal dispersion measurement, the more flow there is, the less able the probe is able to retain heat in the flow stream.

“Flow sensors” are a generic term for flow meters, flow switches, and flow indicators because they are all, in a sense, sensing (or detecting) flow.

Thermal flow sensors are primarily used for laminar-flowing gas in closed systems, although liquids can also be measured and monitored by thermal mass flow sensors.

Thermal mass flow meter come in both inline and insertion models. Insertion type probes offer an easier installation process for existing systems in that it only requires a hole to be made in the pipe versus fully severing the pipe to install an inline flow meter. Insertion models also typically offer less pressure drop than inline thermal mass flow meters. They also offer a more economical option for large pipe diameters than inline meters.

For insertion type flow meters, it is important to note that flow velocity is not uniform across the pipe section and for accurate measurements, the measuring probe must be positioned correctly within the pipe circumference. The tip must be completely covered by the media and the pipe should be full and bubbles should be avoided.

Insertion probes require little to no maintenance and only need to be checked if the media is known to create deposits on the probe.

Unlike some other flow meter technology types, thermal dispersion technology is available as a “stand-alone” flow switch. Many thermal mass flow meters will also offer integral or optional switching capabilities if flow rate measurement is also essential to the process.

• Compatible with many gases
• Can be used with saturated gases
• Accurate and repeatable over a wide range
• No moving parts to wear or break
• Low maintenance
• Long operational life
• Designed for heavy-duty industrial use
• High flow rate sensitivity
• High turndown ratios, up to 100:1
• More economical than other mass flow technology types (Coriolis)
• Unaffected by changes in pressure, temperature, and density
• Quick reaction time to changes in the flow
• Lower installation cost, no extra instrumentation is needed
• Inline models and insertion models for large pipe diameters
• Straight pipe runs can be reduced using dual plate flow conditioning

• Calibration is specific to a particular gas or mixture and accuracy decreases if changed
• Typically only for clean, non-abrasive media
• Thermal properties of the media must be known
• Initially the cost can be higher than other technology types

• Oil & Gas, Upstream and Downstream
• Refineries
• Petrochemicals
• Power Plants
• Nuclear Plants
• Clean Energy
• Landfill Gas Recovery, Biogas
• Flare Gas
• Stack or Flue Gas
• Aeration
• Wastewater Treatment
• Food & Beverage
• Mining & Metals
• Manufacturing
• HVAC
• Pharmaceuticals
• Marine Industry
• Consumption & Supply Monitoring
• Leak Detection
• Monitoring Distribution Networks
• Mixing & Blending of Gas
• Burner or Boiler Feed Control

• Compressed Air
• Carbon Dioxide
• Argon
• Nitrogen
• Oxygen
• Natural Gas
• Biogas
• Air